Device for transmitting motion between the rotor of a synchronous permanent-magnet motor and the working part, having an increased free rotation angle

ABSTRACT

A device for transmitting motion between the rotor of a synchronous permanent-magnet motor and the working part, having an increased free rotation angle, which comprises at least two motion transmission couplings which mutually cooperate in a kinematic series. Each coupling is constituted by at least one driving element which is eccentric with respect to the rotation axis and is rigidly coupled to a first component of the motion transmission system and by at least one driven element, which is also eccentric with respect to the rotation axis and is rigidly coupled to the component arranged kinematically after the preceding one. The angle covered by the elements of each coupling is, as a whole, less than a round angle. The intermediate components of the kinematic transmission have both a driven element and a driving element for receiving the motion from the preceding one and transmitting it to a subsequent one.

BACKGROUND OF THE INVENTION

[0001] This is a continuation-in-part of U.S. patent application Ser.No. 09/423,544, filed Nov. 12, 1999.

[0002] The present invention relates to a device for transmitting motionbetween the rotor of a synchronous permanent-magnet motor and theworking part having an increased free rotation angle.

[0003] It is known that electric motors with permanent-magnet rotor havea structural layout which includes a stator, with an electromagnetconstituted by a lamination pack and by associated windings, and a rotorwhich is arranged between two poles formed by the stator and is axiallycrossed by a shaft which is rotatably connected to a supportingstructure.

[0004] It is also well-known that the higher the inertia of the loadapplied to a synchronous motor, the more difficult it is to start saidmotor.

[0005] Starting in fact occurs as a transient process in which thedirection of rotation, the speed and the current change untilsynchronous operation is achieved.

[0006] During this transient process the rotor oscillates due to thealternating magnetic field produced by the stator, which by inducing atorque on the permanent-magnet rotor tends to move said rotor into aposition in which the magnetic field of said rotor is aligned with thestator field.

[0007] If, during this hunting, the rotor acquires enough kinetic energyto move slightly out of its alignment position, it undergoes a furtheracceleration which makes it turn slightly further, and so forth, untilsynchronous operation is achieved.

[0008] For an equal power level, the extent of the oscillations producedin the rotor increases as the inertia of the applied load decreases;accordingly, the rotor is able to accelerate, acquiring a speed whichallows it to synchronize with the alternating field of the stator.

[0009] If instead the inertia of the load is significant, the extent ofthe oscillation of the rotor is limited and synchronous operation cannotbe achieved.

[0010] As the inertia of the load increases, the extreme situationoccurs in which after power has been supplied to the stator the rotorcannot even begin its oscillation, i.e., it remains motionless in itsequilibrium position.

[0011] When the inertia of the load is not too high with respect to thepower of the motor (for example the impeller of a centrifugal pump),couplings of the mechanical type are currently widely used; saidcouplings are inserted between the load and the rotor and allow saidrotor to oscillate freely, during starting, through a certain rotationalangle.

[0012] This is the case of so-called toothed couplings, in which a firstdriving tooth is eccentric with respect to the rotation axis and isrigidly coupled to the rotor, while a second driven tooth is alsoeccentric with respect to the rotation axis and is rigidly coupled tothe load.

[0013] In this manner, during the starting transient the rotor isdisengaged from the inertia of the load and this makes it easier toachieve synchronous operation.

[0014] Accordingly, there is a free rotation through a certain angle(usually 180 sexagesimal degrees) followed by impact when the load isengaged, providing a direct connection between the load and the rotor,which are in practice rigidly coupled during operation.

[0015] Therefore the free rotation transient allows the motor to reachthe synchronous state and to develop a torque which allows it toovercome the starting moment of inertia of the load.

[0016] After this starting transient, the torque, and therefore thepower, required in the steady state is very often far lower than thestatic torque.

[0017] However, there are applications in which the moment of inertia ofthe load is so high (for example the impeller of a centrifugal pump usedas a washing pump in dishwashers) that even the above mentionedcouplings are unable to start it unless the motor is oversized to thepoint of being excessively expensive to manufacture and use and istherefore of no interest for the user.

[0018] As the inertia and resisting torques increase, the generatedstatic torque must in fact also increase, with the obvious limits posedby the maximum stator flux allowed by permanent magnets, on penalty ofdemagnetizing them, and by the ability of the active components (ironand copper) to dissipate the temperatures generated due to the highcirculating currents that occur even after the transient starting stephas ended.

[0019] A further consequence is the high level of vibration generateddue to the angular torque oscillations caused by a disproportionatechoice of motor size in order to be able to produce the torque requiredfor starting.

[0020] The effect of these oscillations, which are produced at everyturn of the rotor, is to produce an instantaneous separation of the twoteeth of the coupling, consequently generating noise.

[0021] The high static torque also makes it difficult to provideappropriate dimensions for the coupling owing to the intense stressesproduced during impact and leads to the generation of excessive noise.

[0022] In such cases, it is thought that one solution for the initialdriving of the load might be, apart from oversizing the motor, toincrease the angle of free rotation of the rotor with respect to theload, i.e., to provide a greater uncoupling of the motor from the loadduring the starting transient.

[0023] This is currently constrained by the materials used for the partsof the coupling, particularly the teeth, which are usually made ofplastics, as well as by the radial dimensions of the rotor, which arenecessarily modest (on the order of a few tens of millimeters), bearingin mind that the impact of one tooth against the other during startingis considerable.

[0024] The interposition of shock-absorbing means, which is sometimesprovided, worsens the situation because said means also require theirown angular extension and accordingly their presence further reduces theavailable free rotation angle.

[0025] It is also known that synchronous permanent-magnet motors arebidirectional; i.e., at power-on the rotor can equally start turningclockwise or counterclockwise.

[0026] While this is not a problem in the case of the actuation ofcentrifugal pumps with radial vanes, it is a considerable limitation forcentrifugal pumps which have a particular vane configuration andaccordingly a single direction of rotation for the impeller.

SUMMARY OF THE INVENTION

[0027] The aim of the present invention is to provide a device fortransmitting motion between the rotor of a synchronous permanent-magnetmotor and the working part which increases the angle of freedom that canbe provided at present.

[0028] Within the scope of this aim, a consequent primary object is toreduce the power level that is currently required in a permanent-magnetmotor for starting because of the above-mentioned problems, bringing itcloser to the power level absorbed in steady-state operation andtherefore reducing the oversizing required for example to start loadshaving a particularly high inertia.

[0029] Another important object is to ensure that the power absorbed bythe load in one direction of rotation is higher than the power that canbe generated by the motor and that the power absorbed in the oppositedirection of rotation is lower than the power that can be developed,achieving loss of synchronization or pitch, blocking and automaticreversal in the first of such directions of rotation and achievingdriving in the opposite direction of rotation, thus determining a singledirection of rotation.

[0030] Another object is to provide a device for transmitting motionwhich is constructively simple and compact.

[0031] Another object is to provide a device for transmitting motionwhich is silent when starting and during operation.

[0032] Another object is to provide a motor which as a whole has a lowconsumption and a low cost.

[0033] This aim, these objects and others which will become apparenthereinafter are achieved by a device for transmitting motion between therotor of a synchronous permanent-magnet motor and the working part,characterized in that it comprises at least two motion transmissioncouplings which mutually cooperate in a kinematic series, each couplingbeing constituted by at least one driving element which is eccentricwith respect to the rotation axis and is rigidly coupled to a componentof the motion transmission system and by at least one driven element,which is also eccentric with respect to the rotation axis and is rigidlycoupled to the component arranged kinematically after the preceding one,the angle covered by the elements of each coupling being, as a whole,less than a round angle, the at least one intermediate component of thekinematic transmission having both a driven element and a drivingelement for receiving the motion from a preceding one and transmittingit to a subsequent one, giving up to a resulting angle, whichconstitutes a free angle of rotation, being given by the sum of theangles of each coupling.

[0034] Advantageously, said motion transmission couplings are toothed,each coupling being constituted by at least one driving tooth, which iseccentric with respect to the rotation axis and is rigidly coupled to acomponent of the motion transmission system, and by at least one driventooth, which is also eccentric with respect to the rotation axis and isrigidly coupled to the component arranged kinematically after thepreceding one.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Further characteristics and advantages of the invention willbecome apparent from the following detailed description of an embodimentthereof, illustrated by way of non-limitative example in theaccompanying drawings, wherein:

[0036]FIG. 1 is a sectional view of a permanent-magnet electric motorcoupled to a centrifugal pump by means of a device according to theinvention in a first embodiment;

[0037]FIG. 2 is an axial sectional view of the rotor, of the device andof the impeller of FIG. 1;

[0038]FIG. 3 is an exploded perspective view of the device of thepreceding figures;

[0039]FIG. 4 is a transverse sectional view of the device of thepreceding figures;

[0040]FIG. 5 is an enlarged-scale axial sectional view of the device,taken along a plane which is perpendicular of the plane of the sectionalview of FIG. 2;

[0041]FIG. 6 is an axial sectional view of a rotor of a motor of thedevice according to the invention in a second embodiment and of animpeller of a centrifugal pump;

[0042]FIG. 7 is an exploded perspective view of the device of FIG. 6;

[0043]FIG. 8 is a transverse sectional view of the device of FIG. 6;

[0044]FIG. 9 is an axial sectional view of a rotor of a motor of thedevice according to the invention in a third embodiment and of animpeller of a centrifugal pump;

[0045]FIG. 10 is an exploded perspective view of the device of FIG. 9;

[0046]FIG. 11 is a transverse sectional view of the device of FIG. 9;

[0047]FIG. 12 is an axial sectional view of a rotor of a motor of thedevice according to the invention in a fourth embodiment and of animpeller of a centrifugal pump;

[0048]FIG. 13 is an exploded perspective view of the device of FIG. 12;

[0049]FIG. 14 is an axial sectional view of the device according to theinvention in a fifth embodiment and of an impeller of a centrifugalpump;

[0050]FIG. 15 is an exploded perspective view of the device and of theimpeller of FIG. 14;

[0051]FIG. 16 is a transverse sectional view of the device of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] With reference to the above FIGS. 1 to 5, said figures illustratea permanent-magnet electric motor, generally designated by the referencenumeral 10, which comprises a stator 11, with a lamination pack 12 andwindings (not shown), and a rotor 14, which is arranged between twopoles formed by said lamination pack 12.

[0053] The rotor 14 is constituted by an annular cylindrical permanentmagnet 16 whereon a plastic element 17 is overmolded, forming aninternal shank 17 a and end flanges 17 b.

[0054] The rotor 14 accordingly has, as a whole, a cylindrical shapewith an axial hole 18 in which a shaft 19 is rigidly inserted.

[0055] The shaft 19 is in turn supported by a cup-shaped element 20 (thechamber that contains the rotor 14) which belongs to a structure 21 forsupporting the entire motor 10.

[0056] The cup-shaped element 20 contains the rotor 14, separating itfrom the stator 11.

[0057] The shaft 19 is rotatably coupled, in the vicinity of the shapedbottom 22 of the cup-shaped element 20, to a bushing 25.

[0058] The seat for an elastomeric ring 26 is formed between the bushing25 and a similar bushing 24 accommodated in the bottom 22.

[0059] Likewise, bushings 27 and 28 are arranged between a closureelement 23 which is arranged at the opposite end, is fixed to thesupporting structure 21 and is crossed by the shaft 19; said bushingsform, between them, the seat for an elastomeric ring 29.

[0060] The bushing 28 is axially crossed by the shaft 19, which canrotate therein.

[0061] A thrust bearing, generally designated by the reference numeral30, is arranged between the bushing 28 and the corresponding flange 17b.

[0062] According to the invention, between the rotor 14, and accordinglybetween the shaft 19 rigidly coupled thereto, and the working part,which in this case is constituted by an impeller 32 of a centrifugalpump generally designated by the reference numeral 33 and coupled to theelectric motor 10, there is a motion transmission device whichcomprises, in this case, two motion transmission couplings whichmutually cooperate in a kinematic series.

[0063] Advantageously, the motion transmission couplings are of thetoothed type and comprise, in an axial hollow body 34 which protrudesfrom the impeller 32 toward the rotor 14 and is closed by a cover 35(preferably, but not necessarily, a hermetic seal is provided inside it,for example by gluing, gaskets, interference fit, ultrasonic welding,etcetera on the outer edge and an elastomeric lip-shaped ring 36 in theregion where the shaft 19 passes), a first tooth 37 which protrudes froma tang 38 which is keyed to the end of the shaft 19.

[0064] The shaft 19 in fact has, on its end, two diametrically oppositeflat regions 19 a by means of which it couples to a complementarilyshaped hole 38 a of the tang 38.

[0065] Axial fixing is achieved by means of elastic hooks 38 b of thetang 38 which enter an annular groove 19 b of the shaft 19.

[0066] The first tooth 37 is arranged eccentrically with respect to theshaft 19, has a limited radial extension and constitutes a driving toothfor a second tooth 39 which protrudes axially from an intermediateannular element 40 which can rotate freely in the hollow body 34 withrespect to the shaft 19 and to said hollow body 34.

[0067] The second tooth 39 is composed of an internal supporting part 39a made of rigid plastics and of two mutually opposite external parts 39b which are overmolded on the preceding one, are made of elastomer andform the contact surfaces.

[0068] The second tooth 39 might of course also be providedmonolithically without overmolding, using for example a hard elastomericmaterial.

[0069] The radial extension of the second tooth 39 affects all of theregion between the tang 38 and the outer wall of the hollow body 34,providing of course clearances which allow free movement or providing aslight interference (achieving a friction engagement) for example with acircumferential elastomeric element, not shown. The second tooth 39 hasan axial extension which allows it to make contact with the first tooth37 and with a third tooth 41 which protrudes radially from the internalwall of the hollow body 34 to the vicinity of the external profile ofthe first tooth 37.

[0070] The second tooth 39 is therefore a tooth which is driven by thefirst tooth 37 and drives the third tooth 41, interacting therewiththrough the elastomeric parts 39 b.

[0071] The angle covered by the assembly constituted by the first tooth37 and by the second tooth 39 is smaller than a round angle and so isthe angle covered by the assembly constituted by the second tooth 39 andby the third tooth 41, while the angle covered by the two assemblytogether is the sum of each assembly ones When the electric motor 10 isactuated and the rotor 14 begins its rotation, the first tooth 37, i.e.,the tooth that is rigidly coupled thereto, starts to rotate; during itsrotation it encounters the second tooth 39 and moves it.

[0072] The second tooth, being moved, then encounters the third tooth 41and therefore at this point the impeller 32, which is monolithictherewith, is turned.

[0073] Conveniently, it is possible to introduce in the hollow body 34 afluid having an adequate viscosity with lubricating, impact-damping andnoise-deadening functions.

[0074] The motion transmission device is thus composed of two couplingswhich mutually cooperate in a kinematic series; a first one of saidcouplings is constituted by a driving element, which is eccentric withrespect to the rotation axis (the first tooth 37) and is rigidly coupledto a component of the intermediate motion transmission system (the rotor14), and by a driven element (the second tooth 39 with the correspondingpart 39 b), which is also eccentric with respect to the rotation axisand is rigidly coupled to the component arranged kinematically after thepreceding one (the annular element 40).

[0075] A second one of these couplings is composed of a driving element(the second tooth 39 with one of its parts 39 b), which is rigidlycoupled to a component of the motion transmission system (the annularelement 40), and of a driven element (the third tooth 41), which isrigidly coupled to the component of the motion transmission system thatis arranged kinematically to follow (in this example the impeller 32).

[0076] In practice it has been observed that coupling in a kinematicseries at least two toothed motion transmission couplings increases theangle of freedom between the rotor and the working part (in this case,the impeller) that can currently be provided, and this has thebeneficial effect of reducing the power currently required for startingin a permanent-magnet motor.

[0077] This advantage accordingly allows to reduce the oversizing thatis currently necessary to start loads having a particularly highinertia, as in the described case of a centrifugal pump.

[0078] By designing the vanes of the impeller with a configuration whichis not radial but has an adequate curvature, the power absorbed by theload (the impeller and the working fluid) in one direction of rotation(the direction in which the impeller has the lowest efficiency) ishigher than the available power of the motor and is lower in theopposite direction of rotation (the direction in which the impeller hasthe highest efficiency).

[0079] In the first case, the impeller 32 loses its synchronization orpitch with respect to the rotor 14, blocks and then automaticallyreverses its motion, while normal driving occurs in the second case.

[0080] Accordingly, a unidirectional motor has thus been obtainedwithout any electric/electronic or mechanical device.

[0081] With reference now to the above FIGS. 6 to 8, a second embodimentof the motion transmission device has a shaft 119 which can rotatefreely with respect to the rotor 114.

[0082] In this case, an axial tang 138 protrudes from the head flange117 b of the rotor 114 and has a first eccentric tooth 137 which has alimited radial extension.

[0083] The axial extension of the first tooth 137 is approximately halfof the axial extension of the tang 138; at leat one annular element 140is arranged on the other half so that it can rotate freely, and a secondtooth 139 protrudes from said annular element; the axial dimensions ofsaid tooth allow it to make contact with the first tooth 137 and with athird tooth 141 which occupies the radial space outside the first tooth137 and protrudes from another annular element 142 which is rigidlycoupled to the shaft 119.

[0084] In this case also, the second tooth 139 has an internalsupporting part 139 a and two external parts 139 b made of elastomericmaterial.

[0085] As an alternative, the second tooth 139 might be monolithic.

[0086] The impeller, now designated by the reference numeral 132, isrigidly coupled to a pin 143 which is keyed in an axial hole thereof;the pin in turn is keyed to the end of the shaft 119 which is insertedin a suitable hole 144 of said pin.

[0087] In this case also, the rotation of the rotor 114 sequentiallyproduces the transmission of motion between the first tooth 137, thesecond tooth 139 and respectively the third tooth 141, which is rigidlycoupled to the shaft 119, and with the impeller 132.

[0088] With reference now to the above FIGS. 9 to 11, a third embodimentof the motion transmission device again has, between the rotor 214, andconsequently between the shaft 219 rigidly coupled thereto, and theworking part, which in this case also is constituted by an impeller 232of a centrifugal pump which is coupled to the electric motor, twotoothed motion transmission couplings which comprise, in an axial hollowbody 234 which is monolithic with the impeller 232 and, in this case, isclosed hermetically by a cover 235 with a lip-shaped gasket 236, twofirst teeth 237 which protrude in diametrically opposite positions froma first annular element 238 which is keyed to the shaft 219.

[0089] The first teeth 237 are arranged eccentrically with respect tothe shaft 219, have a limited radial extension and constitute drivingteeth for two second teeth 239 which protrude axially from a secondannular element 240 which can rotate freely in the hollow body 234 withrespect to the shaft 219 and to said hollow body 234.

[0090] The second teeth 239 also are diametrically mutually opposite.

[0091] The first teeth 237 are arranged axially in offset positions andthe second teeth 239 are shaped so as to have parts 239 a which protruderadially so as to affect all of the region between the tang 238 and theexternal wall of the hollow body 234, providing of course clearanceswhich allow free movement or providing slight interference, producing afriction engagement, for example with a circumferential elastomericelement, not shown.

[0092] The second teeth 239 have an axial extension which allows them toalso make contact with two third teeth 241 which are also diametricallyopposite and protrude radially from the internal wall of the hollow body234 in axially offset positions.

[0093] The second teeth 239 are therefore teeth which are driven by thefirst teeth 237 and drive the third teeth 241.

[0094] In this case, the parts, and therefore the masses, that rotateare arranged symmetrically with respect to the shaft 219 and thereforerotation is balanced.

[0095] With reference now to the above FIGS. 12 and 13, a fourthembodiment of the motion transmission device has, between the shaft 319and the working part, in this case also constituted by an impeller 332of a centrifugal pump, four toothed motion transmission couplings whichcooperate with each other in a kinematic series.

[0096] Advantageously, two motion transmission couplings are arranged inan axial hollow body 334 which protrudes from the impeller 332 towardthe rotor 314 and is closed by a cover 335, preferably so that ahermetic seal is formed inside it and therefore for example by gluing orby means of the other methods already mentioned, on the outer rim, and alip-shaped ring 336 in the region where the shaft 319 passes.

[0097] In the hollow body 314 there is therefore a first tooth 337 whichprotrudes from a tang 338 which is keyed to the end of the shaft 319.

[0098] The first tooth 337 is arranged eccentrically with respect to theshaft 319, has a limited radial extension and constitutes a drivingtooth for a second tooth 339 which protrudes axially from an annularelement 340 which can rotate freely in the hollow body 334 with respectto the shaft 319 of said hollow body 334.

[0099] The second tooth 339 is composed of an internal supporting part339 a made of rigid plastics and of two mutually opposite external parts339 b which are overmolded on the internal part, are made of elastomericmaterial, and form the contact surfaces.

[0100] In this case also it is possible to provide the second tooth 339monolithically.

[0101] The radial extension of the second tooth 339 affects all of theregion between the tang 338 and the external wall of the hollow body334, providing of course clearances which allow free movement orproviding a slight interference, producing a friction engagement, forexample with a circumferential element made of elastomeric material, notshown.

[0102] The second tooth 339 has an axial extension which allows it tomake contact with the first tooth 337 and with a third tooth 341 whichprotrudes radially from the internal wall of the hollow body 334 to thevicinity of the external profile of the first tooth 337.

[0103] The second tooth 339 is therefore a tooth which is driven by thefirst tooth 337 and drives the third tooth 341, interacting with them bymeans of the elastomeric parts 339 b.

[0104] The angle covered by the assembly constituted by the first tooth337 and the second tooth 339 is smaller than a round angle and so is theangle covered by the assembly constituted by the second tooth 339 and bythe third tooth 341.

[0105] The motion transmission device, in this case, again has an axialtang 342 which protrudes from the head flange 317 b of the rotor 314 andhas a fourth eccentric tooth 343 which has a limited radial extension.

[0106] The axial extension of the fourth tooth 343 is approximately halfof the axial extension of the tang 342; an annular element 344 isarranged on the other half so that it can rotate freely, and a fifthtooth 345 protrudes from it; the axial extension of the fifth tooth issuch that it can make contact with the fourth tooth 343 and with a sixthtooth 346 which occupies the radial space outside the fourth tooth 343and protrudes from another annular element 347 which is rigidly coupledto the shaft 319.

[0107] The fifth tooth 345 has an internal supporting part and twoexternal parts made of elastomeric material which are not shown in thefigures.

[0108] In this case, the rotation of the rotor 314 produces, insequence, the transmission of motion between the fourth tooth 343, thefifth tooth 345 and then the sixth tooth 346, which is rigidly coupledto the shaft 319.

[0109] The shaft begins its rotation and the first tooth 337, the onethat is rigidly coupled thereto, then begins to rotate and encounters,in its rotation, the second tooth 339, moving it.

[0110] At The second tooth, being driven, then encounters the thirdtooth 341 and then at this point the impeller 332, which is monolithicwith respect to the third tooth, is turned. In the particular embodimentjust described it is disclosed that one of the objects of the presentinvention is to provide a free angle between the rotor and the externalload as large as needed, in order to start and synchronise the permanentmagnet synchronous motor with the external load (in this particularexample a pump impeller). The present invention teaches to use aconvenient number (in this specific example two) of annular elementswhich can freely rotate, likewise elements 340 and 344 as in thisspecific example, in order to obtain a free angle of rotation, beinggiven by the sum of the angles of each particular coupling elements.

[0111] With reference to FIGS. 14, 15 and 16, a fifth embodiment of amotion transmission device has, between the shaft 419 and the workingpart, which in this configuration also is the impeller 432 of acentrifugal pump, two motion transmission couplings which mutuallycooperate in a kinematic series.

[0112] In this case, such couplings are arranged inside an axial hollowbody 434 which protrudes from the impeller 432 toward the rotor, whichis not shown in the above figures for the sake of simplicity, and isclosed by a cover 435.

[0113] Preferably, said cover 435 closes said hollow body 434 by gluing,ultrasonic welding, or other methods, so as to ensure hermetic tightnessinternally.

[0114] In the region where the shaft 419 passes through cover 435 thereis a lip-shaped ring 436 made of elastomeric material.

[0115] Inside the hollow body 434 there is a first tooth 437 whichprotrudes from a tang 438 which is keyed to the end of the shaft 419.

[0116] Such end in fact has two diametrically opposite flat regions 419awith which a complementarily shaped hole 438 a of the tang 438 mates.

[0117] The first tooth 437 is eccentric with respect to the shaft 419and has a limited axial extension; it constitutes a driving tooth for asecond tooth 439 which protrudes axially from an annular element 440which can rotate freely, inside the hollow body 434, with respect to theshaft 419.

[0118] The second tooth 439 has an internal supporting part 439 a madeof rigid plastics which is monolithic with the annular element 440,which in this case has a substantially cylindrical structure, and isembedded in the remaining part 439 b made of elastomeric material, whichis overmolded on the preceding one and forms the contact surfaces.

[0119] In this case also, the third tooth 439 might be providedmonolithically.

[0120] The radial extension of the second tooth 439 affects all of theregion between the tang 438 and the internal wall of the hollow body434.

[0121] Clearances are of course provided which allow free motion or, asan alternative, there is a slight interference which produces frictionengagement, for example by resorting to a circumferential element madeof elastomeric material which is not shown for the sake of simplicity.

[0122] The second tooth 439 has an axial extension which allows it tomake contact with the first tooth 437 and with a third tooth, nowdesignated by the reference numeral 441, which protrudes axially fromsaid cover 435 from a position which is proximate to the outer profileof the face that is directed toward the inside of the hollow body 434.

[0123] In particular, the axial extension of the third tooth 441 is suchthat it can make contact only with the elastomeric external parts 439 bof the second tooth 439 but cannot make contact with the first tooth437.

[0124] The second tooth 439 is accordingly driven by the first tooth 437and in turn drives the third tooth 441 by means of the elastomeric parts439 b.

[0125] The angle covered by the assembly constituted by the first tooth437 and by the second tooth 439 is less than a round angle and so is theangle covered by the second tooth 439 together with the third tooth 441.

[0126] When the motor is started, the first tooth 437 therefore startsto rotate rigidly with the rotor until it encounters, during itsrotation, the second tooth 439, which it moves.

[0127] The second tooth therefore starts to rotate concordantly with therotor until it encounters the third tooth 441, which is rigidly coupledto the hollow body 434 and accordingly to the impeller 432, which istherefore turned. If the coupling between the permanent magnetsynchronous motor and the external load requires an increased freerotation angle in order to start and synchronise the motor with theload, this can be obtained in this particular embodiment by inserting aconvenient number of annular freely rotating elements like element 440between the motor and the applied load.

[0128] In practice it has been observed that in all its embodiments theinvention has achieved the intended aim and objects.

[0129] The invention thus conceived is susceptible of numerousmodifications and variations, all of which are within the scope of theinventive concept.

[0130] All the details may also be replaced with other technicallyequivalent elements.

[0131] In practice, the materials employed, so long as they arecompatible with the contingent use, as well as the dimensions, may beany according to requirements.

[0132] The disclosures in Italian Patent Application No. PD98A000058from which this application claims priority are incorporated herein byreference.

What is claimed is:
 1. A device for transmitting motion between therotor of a synchronous permanent-magnet motor and the working part,comprising at least two motion transmission couplings which mutuallycooperate in a kinematic series, each coupling being constituted by atleast one driving element which is eccentric with respect to therotation axis and is rigidly coupled to a component of the motiontransmission system and by at least one driven element, which is alsoeccentric with respect to the rotation axis and is rigidly coupled tothe component arranged kinematically after the preceding one, the anglecovered by the elements of each coupling being, as a whole, less than around angle, the intermediate components of the kinematic transmissionhaving both a driven element and a driving element for receiving themotion from a preceding one and transmitting it to a subsequent one, aresulting angle, which constitutes a free angle of rotation, being givenby a sum of the angles of each coupling.
 2. The device according toclaim 1, wherein said motion transmission couplings are toothed, eachcoupling being constituted by at least one driving tooth, which iseccentric with respect to the rotation axis and is rigidly coupled to acomponent of the motion transmission system, and by at least one driventooth, which is also eccentric with respect to the rotation axis and isrigidly coupled to the component that kinematically follows thepreceding one.
 3. The device according to claim 2, wherein two couplingswhich mutually cooperate in a kinematic series, a first one of saidcouplings being constituted by a first tooth which is rigidly coupled tothe rotor of a motor and by a second tooth which is rigidly coupled toan annular element which can rotate freely with respect to said rotor, asecond one of said couplings being composed of said second tooth and athird tooth which is rigidly coupled to the working part.
 4. The deviceaccording to claim 2, wherein two couplings which mutually cooperate ina kinematic series, a first one of said couplings being constituted bytwo first teeth which are rigidly coupled to the rotor of a motor indiametrically opposite positions, and of two second teeth which arerigidly coupled, likewise in diametrically opposite positions, to anannular element which can rotate freely with respect to said rotor, asecond one of said couplings being composed of said second teeth and oftwo third teeth which are also diametrically opposite and are rigidlycoupled to the working part.
 5. The device according to claim 2,comprising four couplings which mutually cooperate in a kinematicseries, a first one of said couplings being constituted by a first toothwhich is rigidly coupled to the free shaft of a motor and of a secondtooth which is rigidly coupled to an annular element which can rotatefreely with respect to said free shaft, a second one of said couplingsbeing composed of said second tooth and of a third tooth which isrigidly coupled to the working part, a third one of said couplings beingcomposed of a fourth tooth which is rigidly coupled to the rotor of saidmotor and of a fifth tooth which is rigidly coupled to an annularelement which moves freely with respect to said free axis, a fourth oneof said couplings being composed of said fifth tooth and of a sixthtooth which is rigidly coupled to said free shaft.
 6. The deviceaccording to claim 3, wherein said motion transmission couplings arearranged in an axial hollow body which is rigidly coupled to saidworking part and is closed by a cover.
 7. The device according to claim6, wherein said first tooth protrudes from a tang which is keyed on theend of said shaft, said first tooth being arranged eccentrically withrespect to said shaft and constituting a driving tooth for said secondtooth which protrudes axially from an annular element which can rotatefreely in said hollow body with respect to said shaft and to said hollowbody, said second tooth having an extension which allows it to makecontact with said first tooth and with said third tooth which protrudesfrom the internal wall of the hollow body.
 8. The device according toclaim 7, wherein said first tooth has a radial extension which partiallyaffects the internal space of said hollow body, the radial extension ofsaid second tooth affecting the region between said tang and theexternal wall of said hollow body, providing clearances which allow freemovement, said second tooth having an axial extension which allows it tomake contact with said first tooth and with said third tooth, said thirdtooth protruding radially from the internal wall of said hollow body tothe vicinity of the external profile of said first tooth.
 9. The deviceaccording to claim 3, wherein said motion transmission couplingscomprise a first tooth which is rigidly coupled to an axial tang whichprotrudes from a tip flange of said rotor, said annular element fromwhich said second tooth protrudes being arranged so as to surround saidshaft and so that it can rotate freely, the extension of said secondtooth being such that it can make contact with said first tooth and withsaid third tooth which protrudes from another annular element which isrigidly coupled to said shaft.
 10. The device according to claim 9,wherein the axial extension of said first tooth partially affects theextension of said tang, the remaining part being affected by saidannular element, from which said second tooth protrudes.
 11. The deviceaccording to claim 9, wherein said third tooth occupies the radial spaceprovided externally with respect to said first tooth.
 12. The deviceaccording to claim 4, wherein said couplings are arranged in an axialhollow body which is rigidly coupled to said working part and is closedhermetically by a cover.
 13. The device according to claim 12, whereinsaid two first teeth protrude in a diametrically mutually oppositeconfiguration from a first annular element which is keyed to the shaftof a motor, said first teeth radially and partially affecting the spaceinside said hollow body, said first teeth constituting driving teeth forsaid two second teeth which protrude axially from a second annularelement which can rotate freely in said hollow body with respect to saidshaft and to said hollow body, said second teeth having an extensionwhich allows them to make contact also with two third teeth whichprotrude radially from the internal wall of said hollow body in theregion left free by said first teeth.
 14. The device according to claim13, wherein said first teeth are arranged in axially offset positionsand in that said second teeth are shaped so as to have parts whoseradial extension affects all of the region between said tang and theexternal wall of the hollow body, providing clearances which allow freemovement, said second teeth having an axial extension which allows themto make contact with said first teeth and with said third teeth whichprotrude radially from the internal wall of said hollow in axiallyoffset positions.
 15. The device according to claim 5, wherein two ofsaid four motion transmission couplings are arranged in an axial hollowbody which protrudes from said working part toward said rotor and isclosed by a cover.
 16. The device according to claim 15, wherein saidfirst tooth protrudes from a tang which is keyed to the end of said freeshaft, said first tooth being arranged eccentrically with respect tosaid shaft and constituting a driving tooth for said second tooth whichprotrudes axially from an annular element which can rotate freely insaid hollow body with respect to said shaft and said hollow body, saidsecond tooth having an extension which allows it to make contact withsaid first tooth and with said third tooth which protrudes from theinternal wall of the hollow body, the other two of said four motiontransmission couplings comprising said fourth tooth, which is rigidlycoupled to an axial tang which protrudes from a tip flange of the rotorof said motor, so as to surround said free shaft, said annular elementfrom which said fifth tooth protrudes being arranged so that it canrotate freely, the extension of said fifth tooth being such that it canmake contact with said fourth tooth and with said sixth tooth whichprotrudes from another annular element which is rigidly coupled to saidshaft.
 17. The device according to claim 2, wherein at least one of saidteeth is composed of an internal supporting part which is made of rigidplastics and of two mutually opposite external parts which areovermolded on the internal part, are made of elastomeric material, andform the surfaces for contact with the other teeth.
 18. The deviceaccording to claim 4, wherein said cover closes said hollow body so thata hermetic seal is provided therein.
 19. The device according to claim1, wherein a slight interference is provided between the mutually movingparts, producing a friction engagement.
 20. The device according toclaim 4, wherein in said hollow body there is a viscous fluid which haslubricating, impact-damping and noise-deadening functions.
 21. Thedevice according to claim 5, wherein said first tooth protrudes from atang which is keyed to the end of said shaft, said first tooth beingarranged eccentrically with respect to said shaft and constituting adriving tooth for said second tooth which protrudes axially from anannular element which can rotate freely in said hollow body with respectto said shaft and to said hollow body, said second tooth having anextension which allows it to make contact with said first tooth and withsaid third tooth which protrudes from the face of said cover which isdirected toward the inside of the hollow body.
 22. The device accordingto claim 21, wherein said first tooth has a radial extension whichpartially affects the internal space of said hollow body, the radialextension of said second tooth affecting the region between said tangand the external wall of said hollow body, providing clearances whichallow free movement, said second tooth having an axial extension whichallows it to make contact with said first tooth and with said thirdtooth, said third tooth protruding radially from a position which isproximate to the external profile of the cover to the vicinity of theexternal profile of said annular element.
 23. The device according toclaim 1, wherein said working part is an impeller with curved vanes of acentrifugal pump.